There is a practical need to extend the useful life of the electrodes associated with oxygen ion conductive solid electrolyte electrochemical cells used in flue gas streams of combustion systems. Power plant flue gases typically include sulfur-containing gases which, when insufficient oxygen is present, can produce adverse sulfiding of electrodes, which typically consist of platinum group metals, such as rhodium, palladium, iridium, platinum, etc., as well as silver. This sulfiding results in increased cell resistance and deteriorating cell operation.
Oxygen ion conductive solid electrolyte electrochemical cells constructed in accordance with the teachings of U.S. Pat. No. 3,400,054, issued Sept. 3, 1968, assigned to the assignee of the present invention, and incorporated herein by reference, typically operate over a temperature range of between approximately 600.degree. C and 1100.degree. to measure the oxygen content of a monitored environment. A platinum group electrode of a typical oxygen analyzer operating in this temperature range in a monitored environment containing an excess of oxygen and small quantities of sulfur compounds will evaporate slowly as an oxide, but the remaining electrode material remains largely that of the platinum group material and remains a useful electrode for an extended period of time.
However, if the flue gas environment contains a of fuel constituent, such as CO or H.sub.2, in an amount in excess of that required for stoichiometric combustion, then sulfur compounds such as H.sub.2 S, COS, or S can form in the flue gas, react with the electrode material, and form sulfides which do not perform well as electrodes. If the monitored environment then returns to an oxidizing atmosphere, i.e., excess oxygen, pure platinum is reformed from the sulfide by reaction with oxygen. However, repeated excursions from oxidizing to sulfiding atmospheres eventually damage the performance of the electrode as manifested by an increase in the polarization resistance due to reduced contact of the electrode with the solid electrolyte material, spalling of the electrode, corrosion around the edge of the electrode, etc.
An oxidizing atmosphere is defined as an atmosphere in which sulfur-containing gases will not react with the sensing electrode material, i.e., platinum or silver, or metal connectors to form a sulfide.